Electro-optic (E/O) modulators are devices used in optical transmission systems for varying the phase or the amplitude of a laser beam. The phase or amplitude variations of the laser beam enable to code data which are then transmitted via the laser beam. Phase modulators may be used in laser beam amplitude modulation systems, for example, modulators of Mach-Zehnder type.
FIG. 1A is a partial simplified reproduction of FIG. 3.1.A of Xiaotie Wu's thesis entitled “High performance optical transmitter for next generation superconducting and data communication”. This drawing shows an example of an E/O amplitude modulator of Mach-Zehnder type. Mach-Zehnder type modulator 1 comprises a first waveguide 3 divided into two waveguides, or arms, 5 and 7 of same length, arms 5 and 7 being subsequently gathered in a waveguide 9. Each of arms 5 and 7 has an E/O phase modulator 11 inserted therein. Waveguides 3, 5, 7, and 9 are made of silicon and are formed from a Silicon On Insulator (SOI) silicon layer.
In operation, an input laser beam is injected into waveguide 3. The input laser beam is then separated into a first secondary laser beam propagating in arm 5 and a second secondary laser beam, of same phase and of same amplitude as the first one, propagating in arm 7. Phase modulator 11 of one and/or the other of arms 5 and 7 enables to introduce a phase shift between the two secondary beams. Thus, when the two secondary beams are gathered in a same output beam in waveguide 9, optical interferences occur, whereby the amplitude of the output beam is modulated.
FIG. 1B is a partial simplified copy of FIG. 3.1.B of the above-mentioned thesis. FIG. 1B is a cross-section view along plane BB of FIG. 1A showing an example of an E/O phase modulator of the Silicon Insulator Silicon CAPacitor (SISCAP) type. Phase modulator 11 comprises a doped single-crystal silicon strip 13 of a first conductivity type. Strip 13 rests on an insulating layer (not shown) itself resting on a substrate (not shown) and forms a portion of an SOI-type layer. Modulator 11 further comprises a doped polysilicon strip 15 of the second conductivity type. A portion 17 of strip 15 covers a portion 19 of strip 13, portions 17 and 19 being separated from each other by an interface layer made of a dielectric material (not shown), which forms a capacitor. Each of strips 13 and 15 is coupled to an electric contact, respectively 21 and 22. The assembly of strips 13 and 15 is encapsulated in a dielectric material (not shown) having a smaller optical index than strips 13 and 15. Portions 17 and 19 thus form a waveguide 23 capable of propagating a laser beam along a direction orthogonal to the plane of the sheet.
The phase modulator is manufactured by deposition and etching of a polysilicon layer on the upper surface of a single-crystal silicon layer coated with an insulating interface layer. As a result, the upper surface of the polysilicon strip and the upper surface of the single-crystal silicon strip are not at the same level. In operation, the charge density in portions 17 and 19 is modified by applying a potential difference between contacts 21 and 22. This results in a modification of the optical index of the waveguide and thus in a phase shift of the laser beam propagating in the waveguide. A modulator of the type in FIG. 1B has various disadvantages, at least some of which are desired to be overcome.